The contemporary design of internal combustion engines must cope with the increasingly stringent regulations on pollutant emissions. Accordingly, automotive engineers strive for designing engines with low fuel consumption and low emission of pollutants, which implies including electronic devices capable of monitoring the combustion performance and emissions in the exhaust gases.
A proper operation of a fuel-injected engine requires that the fuel injectors and their controller allow for a timely, precise and reliable fuel injection. Indeed, it is well known that problems arise when the performance, or more particularly the timing, and the quantity of fuel delivered by the injectors diverge beyond acceptable limits. For example, injector performance deviation or variability will cause different torques to be generated between cylinders due to unequal fuel quantities being injected, or from the relative timing of such fuel injection.
In order to take into account the flow specificities of a fuel injector, it has been proposed to associate to a given fuel injector a number of performance parameters thereof. These performance parameters are, e.g., encoded in a bar code applied to the injector, so that the flow performance parameters can be retrieved by a bar code scanner at the time of installation in the engine and transferred to the engine control unit (ECU). Such method for fuel injector parameters installation is for example described in U.S. Pat. No. 7,136,743.
Another method of fuel injector installation has been disclosed in WO2011/073147, which uses a segmented master flow curve (fuel quantity vs. pulse width, i.e. injector actuation time). Each fuel injector to be installed in the engine is provided with specific fuel injector parameters in a machine-readable format, and these parameters are transferred to the engine ECU. Fitting information, preferably coefficients for a characteristic equation attributed to each respective segment of the master flow curve, are contained in these fuel injector specific parameters.
It may be further noted that the problem of injector variability is particularly acute when injecting small fuel quantities. Therefore, fuel injector behavior in the so-called “ballistic” domain has been studied in detail.
A first approach in the reliable injection of small fuel quantities is to take into account injector response delays at opening and closing.
Besides, in order to take into account the drift of fuel injectors due to ageing, learning strategies have been developed to detect the injector specific “minimum drive pulse” (MDP), i.e. the minimum injector actuation time required for the smallest injection amount to occur. It may be noted that the length of an injector drive pulse, which influence the injector open time, is herein referred to as pulse width (PW). Commonly used strategies for determining the MDP are APC (accelerometer pilot control) and speed pilot control (SPC).
APC relies on the detection of the start of the combustion by means of a knock sensor. A pilot injection is set in a particular condition in such a way as to give a detectable signal about a known timing. The pilot fuel quantity is progressively increased until the fuel starts burning. The corresponding noise is detected and processed to compute the MDP.
The MDP detection by the SPC strategy relies on an increase of average speed due to the learning pilot injection, whereby a difference of speed exists between two consecutive injector events of the learning injector and the previous injector. The MDP is then detected as pulse width for which the speed difference exceeds a predetermined threshold.
Hence, while modern diesel injection strategies use a pilot injection, before the main injection, to reduce knocking, the efficiency of this pilot injection still depends on the accuracy of the injected fuel quantity. It is therefore desirable to be capable of properly controlling pilot injections, which implies monitoring the pilot fuel combustion.
US 2009/0292447 discloses a method for controlling fuel combustion in an internal combustion engine, wherein fuel injection is divided in a main injection and a pilot injection in advance of the main. The engine controller determines an amount of heat release based on the pilot injection, and corrects the fuel injection amount of the pilot injection based on the determined amount of heat release. As it is known in the art, the amount of heat release is proportional to the injected fuel quantity and can be determined from the in-cylinder pressure during combustion.
As a matter of fact, US 2009/0292447 discloses a closed-loop control of the pilot injection based on the amount of heat release. The control is thus made such that the amount of heat release based on the pilot converges to a target value. However, the approach described in US 2009/0292447 appears to be difficult to implement in practice, in particular where modern emission reduction strategies seek to minimize the time intervals between injected amounts within a same combustion cycle. Accordingly, when there are e.g. 3 injections (two pilots and one main) within an engine combustion cycle, the respective combustion events tend to overlap and determining the pilot contribution is difficult.